This invention relates to an automatic focusing apparatus suitable for incorporation in, for example, a video camera.
Video cameras are provided with means for automatically adjusting the focus so as to enhance the operability, and various automatic focusing means have been proposed hitherto. An example of such means is disclosed in, for example, U.S. Pat. Nos. 3,435,744, 3,442,193 and 3,443,502. The apparatus disclosed in the U.S. patents cited above includes light emitting means and light receiving means. A beam of light emitted from the light emitting means is directed toward a subject, and a portion of the light beam reflected from the subject is received by the light receiving means. A sensor in the light receiving means senses the position of irradiation with the reflected light beam thereby adjusting the focus.
The practical structure of such a prior art automatic focusing apparatus will be described with reference to FIG. 1. In FIG. 1, the reference numerals 1, 2, 3, 4, 5, 6, 7, 8 and 13 designate a light projecting lens, a light emitting element, a light receiving lens, a light receiving element, a focus ring, a cam provided on the rear end of the focus ring, a link mechanism, a resilient member, and a fixed cylinder including lenses VE and RE respectively.
This automatic focusing apparatus is composed of a light emitter part including the light projecting lens 1 and the light emitting element 2, a light receiver part including the light receiving lens 3 and the light receiving element 4 provided with a half-split sensor, and a light-receiving element driver part including the focus ring 5 having the cam 6, the link mechanism 7, and the resilient member 8.
The focus ring 5 is coupled with the fixed cylinder 13 by multiple screws thereby being rotated by the motor M and moving forward and backward directions in accordance with its rotation. A focusing lens element LE functioning as an accurately focusing element is incorporated in the focus ring 5, and a relay lens element RE and a variable lens element VE having its optical axis aligning with that of the lens element LE are disposed within the fixed cylinder 13. An image of a subject 11 is focused by the combination of the lens element LE, the variable lens element VE and the relay lens element RE on an image-receiving surface of a solid-state image pickup element SE (or a camera tube).
The light emitter part and the light receiver part are so disposed that the optical axes of the light projecting lens 1 and light receiving lens 3 are spaced apart by a predetermined distance l and are parallel to each other. The focus ring 5 is rotated by a motor M. The link mechanism 7 is disposed between the focus ring 5 and the light receiving element 4 so that the rotation of the focus ring 5 can be transmitted to the light receiving element 4 to cause linear movement of the light receiving element 4 in a direction as shown by the arrow x. This link mechanism 7 includes a first lever having a length l.sub.1 and a second lever having a length l.sub.2, and the connection point of these levers is rotatably pivoted to a stationary member of the apparatus. The other end of the first lever having the length l.sub.1 is in contact with the cam 6 provided on the rear end of the focus ring 5, and the other end of the second lever having the length l.sub.2 is in contact with a portion of the light receiving element 4. The light receiving element 4 is resiliently biased by the resilient member 8 so that the other end of the second lever of the link mechanism 7 can be brought into intimate contact with the portion of the light receiving element 4 without any clearance therebetween.
Because of the above structure, the rotating movement of the focus ring 5 is converted by the combination of the cam 6 and the link mechanism 7 into a linear movement in a direction as shown by the arrow z, thereby causing the linear movement of the light receiving element 4 in the direction of the arrow x. The stroke of the linear movement of the focus ring 5 due to its rotation is magnified by the cam 6, so that the light receiving element 4 can be positioned with very high accuracy.
In operation, a beam of light emitted from the light emitting element 2 is directed through the light projecting lens 1 toward and onto the subject 11 located at a position spaced by a distance y from the light receiving lens 3. The half-split sensor provided on the light receiving element 4 is composed of two sensing elements S.sub.1 and S.sub.2 joining each other, as shown in FIG. 1B when a reflected beam spot SP impinges on the junction (the splitting line) between the two sensing elements S.sub.1 and S.sub.2, that is, when the beam spot SP impinges uniformly on the two sensing elements S.sub.1 and S.sub.2 at the junction, the two sensing elements S.sub.1 and S.sub.2 generate output signals which are equal to each other. The reflected beam spot SP from the subject (object) 11 passes through the light receiving lens 3 to be focused on the half-split sensor 4 provided on the light receiving element.
When the reflected beam spot SP does not impinge uniformly on the junction (the splitting line) between the two sensing elements S.sub.1 and S.sub.2 of the half-split sensor, that is, when the reflected beam spot SP impinges on only one of the two sensing elements S.sub.1 and S.sub.2, the focus ring 5 is rotated by the motor M to cause movement of the light receiving element 4 in the direction of the arrow x until the reflected beam spot SP impinges uniformly on the two sensing elements S.sub.1 and S.sub.2 at the junction therebetween. The motor M ceases to rotate when the reflected beam spot SP impinges uniformly on the two sensing elements S.sub.1 and S.sub.2 at the junction therebetween, as shown in FIG. 1. With the rotation of the focus ring 5, the focusing lens LE in the photographic optical system TL of the video camera moves in the direction of the optical axis. The optical system TL of the video camera is capable of accurate focusing when finally the reflected beam spot SP impinges uniformly on the junction between the two sensing elements S.sub.1 and S.sub.2 of the half-split sensor of the light receiving element 4.
The stroke x of movement of the light receiving element 4 is a function of the spacing l between the optical axis of the light projecting lens 1 and that of the light receiving lens 3, the focal distance f of the light receiving lens 3, and the distance y from the light receiving lens 3 to the subject 11, as follows: EQU x=fl/y (1)
Also, the stroke x of movement of the light receiving element 4 is related to the stroke z of movement of the focus ring 5 having the cam 6 and the lengths l.sub.1, l.sub.2 of the respective levers of the link mechanism 7, as follows: EQU x=l.sub.2 z/l.sub.1 (2)
In order to reduce the overall size of the video camera, it is necessary to reduce the size of the light emitter part and that of the light receiver part. For this purpose, it is necessary to shorten the spacing l. However, shortening the spacing l results in a corresponding decrease of the movable stroke x of the light receiving element 4 in view of the equation (1). It is therefore necessary to decrease the length l.sub.2 of the second lever of the link mechanism 7 or the movable stroke z of the focus ring 5 having the cam 6 or to increase the length l.sub.1 of the first lever of the link mechanism 7, in the equation (2). However, since the cam 6 is provided for magnifying the stroke z of linear movement of the focus ring 5 thereby positioning the light receiving element 4 with very high accuracy as described above, it is objectionable to decrease the movable stroke z of the focus ring 5 having the cam 6. Further, decreasing the length l.sub.2 of the second lever of the link mechanism 7 is limited from the aspect of design.
Also, when the length l.sub.1 of the first lever of the link mechanism 7 is increased, the light receiver part is correspondingly spaced apart from the photographic lens (i.e. objective lens), resulting in such a defect that the overall size of the video camera increases inevitably or the reflected beam spot from the subject 11 deviates greatly in the range finder depending on the distance y from the light receiving lens 3 to the subject 11. Thus, when the link mechanism 7 is incorporated, it is difficult to decrease the spacing (the base length) l between the light emitter part and the light receiver part, and, also, the structure of the automatic focusing apparatus becomes quite complex.